Tuesday, December 29, 2009

787 Wing - A New Twist on Structural Engineering?

(Thanks to Jon Ostrower, at http://www.flightglobal.com/blogs/flightblogger/, for the graphic!)

I was a bit relieved to read comments indicating other folks also thought the 787 wing had an unusual amount of wing deflection during the first flight video.

In particular, the aft view of ZA001's climb-out seemed to indicate an amazing curvature/bending of the wing-

Reuters video of ZA001 First Flight Takeoff
0:18 great T-33 fun- swooping in for chase during takeoff roll
0:37 front view of wing bending
0:53-1:03 amazing (apparent) wing deflection

Now, camera angles can play tricks with our perception (Bonus Prize to those who have already identified a contributing effect- see first post of this new thread).

But discounting that, I did some surfing about to investigate the 787 wing. (Unfortunately, merely on the web, not long boarding as our friend Baron is doing off the coast of Brazil this holiday season- rats! :)

It turns out, Jon Ostrower, who runs the great FlightBlogger website, has been examining this topic for some time- here is his July 30, 2008 article "A Closer Look at 787 Wing Flex" (gulp! guess we're catching up a bit! :). My special thanks to Jon for letting me borrow the graphic above and to link in to his article.

As one of Jon's commenters noted, "One wag joked(?) that the only reason MCboeing put larger windows on the 787 so that the passengers would not get concerned when the lost sight of the wingtip...."

Julius noted the landing gear issues on the first flight of the second flight test article (ZA002), which also sent me surfing- (sounds like the nose gear initially only deployed 75 degrees- the crew did an "emergency" extension (which some say, is not that unusual for first flights or after maintenance) to get it down and locked, but the situation resulted in more video coverage than usual of the landing, which shows some fairly substantial wing flex too- looks like coincident with the ground spoilers deploying- guess those things really do have a big effect on brake effectiveness (among other things, "runway friction coefficient" x "actual weight on wheels") and landing lengths. This also prompted a review of the ZA001 first landing, which shows similar "flapping" during landing. (More correctly, relaxation as the wings unload, but heck, "flapping" sounds more spectacular).

ZA001 First Flight & First Landing (check out last few seconds of video)

ZA002 First Landing

With Jon's blog substantiating our observations, let's investigate this wing flex stuff!

As well commented upon, the 787 is a constructed with composites. I'm no structural engineer, and I'm sure there are many subtle variations, but it seems the terminology of choice is Carbon Fiber Reinforced Polymers, or CFRP.

Trying to find specifications for CFRP was one of the most frustrating experiences I've had on the web- quoted strengths varied widely, and most of the reference material is only available by purchasing trade journal reprints. (Given the wide variance of the open source material, I did not have confidence I would find a definitive answer with the journal articles). There was also some information about reinforcing concrete with CFRP- which while intriguing, I thought not too applicable for our purposes. (It seems CFRP makes a dandy "wrap" for concrete cylinders and beams that make up highway supports- I think after the Northridge earthquake in the Los Angeles area, circa 1994, many of the freeway overpass supports were reinforced with this fabric).

But, with little confidence in any alternative, I resorted to our steadfast reference, Wikipedia, which pointed me to "AS4", which seems to be a representative aerospace CFRP,
Hexcel AS4.

For comparison with conventional, shall we say, "non-disruptive", aluminum construction, I found a variety of sources- it seems like 7075T6 is a good representative material,
Alcoa 7075 fact sheet.

For our study, I used the Hexcel AS4 datasheet "Typical 350ºF Epoxy Composite Properties (at Room Temperature)" values, and the compression values, rather than tension, as bending loads create both conditions, on the "near" and "far" side of the article subject to the bending load. (The compression values used in this study are somewhat lower than the "flexure" strength listed in the AS4 table, so this represents the conservative case- I suspect the flexure values are for tension side of a loaded object, such as when working with prestressed beams, e.g., reinforced concrete and such- any stress engineer types out there?).

From Wikipedia, the above references, and a few other scribbled notes over the pat couple of days, I pieced together this table. (It's in metric units. I confess, rather than demonstrating my enlightenment, it reflects my laziness in not converting to "English" units. Well, make that "American" units, as even the British use the metric system... But since we'll be doing relative comparisons, the unit's won't matter- one less thing for me to goof up! :)

So here's the deal- we'll be using three metrics (so to speak!) of performance:

Young's Modulus, which is "stiffness": the amount of load (force per cross section area) divided by the resultant strain (axial deflection per reference length).

Yield Strength, the load (force per cross section area) that produces permanent deformation in aluminum, or damaged fibers in composites. (Note: this is slightly different than "ultimate" load, which is the "breaking point"- complete failure- but we will assume the airplane is kept out of the damage region).

Density, the mass per volume (perhaps there is a slight difference between "denseness" and "density" ... :)

So, here we go, the Mr. Science overview (these numbers are approximate, and "Your Mileage May Vary", but seemed to be the most typical values I could find):

Material ....... Young's Modulus ...... Yield Strength ...... Density
7075-T6 ........ 69 GigaPascals ....... 430 MegaPascals ..... 2700 Kg/m^3
CFRP (60%) .... 128 GigaPascals ...... 1530 MegaPascals .... 1550 Kg/m^3

Note: the CFRP properties are for "along fiber" loads, not cross-loads, which are markedly lower (more about that later- #1), but for bending, this is appropriate.

A higher value of Young's modulus means a "stiffer" structure- and CFRP is about twice as stiff as aluminum. (So what's up with this goofy-looking 787 wing? Calm down- let's continue! :) At least this "reinforces" our stereotype of "composites being better than aluminum".

A lower density value is also a good thing in general, and CFRP once again demonstrated it lives up to the stereotype expectations of composite's superiority over aluminum- the latter being about twice as "heavy" (dense) as CFRP. (Well, 2700/1550 = 1.74 to be "exact". Hmmm, so far, things are distressingly stereotypical, rather than disruptive!)

A higher yield strength is also a good thing. Again, CFRP follows stereotypical expectations, with a nearly four-fold advantage over aluminum. (Okay, 1530/430 = 3.56 or so, but hey- this is "ball park stuff"! :)

So, all the material properties would seem to be just as we would expect- so why all that wing bending?? Let's consider the primary design criteria for a wing: weight and strength. Does stiffness matter? Uh, well, er, "it ought to". But for now, let's say no (we'll come back to that one also #2!)

Let's look at "strength"- what it takes to keep the wing from "breaking" (Although strictly speaking, we will use yield- the point of permanent deformation- rather than breaking strengths). To handle a given load (bending load, which is converted to axial tension and compression, in the upper and lower wing skins, and upper and lower web caps of the spars), CFRP is about four times (3.56) as strong as aluminum. So, we can use one-quarter (28%) as much, to get the same strength (resistance to yielding or damage). There are two ramifications of this- one is obvious, the other not-so-obvious.

a) Obviously, there's a tremendous weight savings! (Ah, more on THAT later #3). And figure CFRP is about half as dense (0.57), the total weight savings would be about four times two: the composite structure would weight roughly 1/8 of the aluminum wing! (Or a bit less roughly, (1/3.56) x 0.57 = 0.16, or about 1/6; More on this later #4, with some real-world adjustments...).

b) Less obviously (until we saw the videos and Jon's graphic at the top), is: DEFLECTION. Since CFRP is -about- four (3.56) times as strong as aluminum, a wing designer can use one-quarter (28%)as much. But the stiffness is "only" twice (128/69 = 1.82) that of aluminum;

SO, "one-quarter the material" x "twice the stiffness"
(Okay, (1530/430) x (128/69) = 0.52 the stiffness = 1.92 the deflection)

MYSTERY SOLVED ! (Yeah! Well, basically...); Viola!, as one public icon of past exuberantly, and famously, (mis-)proclaimed. (Which icon? I'm not so sure :)

NOW, back to those pesky "later" items mentioned above (#1, 3 & 4; #2 follows later):

#1) "CFRP properties are for "along fiber" loads, not cross-loads, which are markedly lower..."

#3) there's a tremendous weight savings

#4) the composite structure would weight 1/6 of the aluminum wing ...some real-world adjustments

All three of these items can be summarized in one discussion: how much additional material is required to compensate for the anisotropic (directionally dependent) properties of fiber reinforced materials, versus the isotropic (universal in all directions) characteristics of aluminum, and most metals for that matter. (There are some metallic structures, particularly crystalline turbine blades, that are not isotropic, but such exceptions are rare- and expensive).

Wings are subject to complex loads (different than "wing loading", weight/area). Obviously, with the shear, bending, and torsion, the load paths are complex, and this is one area the anisotropic nature of composites can create problems. Consider just how unidirectional composite strength can be: the shear strength of a single-direction layup is only 81 MPa for 90-degree cross load, or a mere 4% of the 0-degree tensile strength os 2205 MPa; and shear strength is only 128 MPa, or 6% of the 0-degree tensile strength. (By comparison, aluminum is equally strong in any axis, and the shear strength of 7075T6 is 331 MPa, or 65 percent of the 503 MPa yield strength in this ASM spec sheet, which is some 20% stronger than the yield strength listed in the Alcoa spec sheet, which did not list shear strength, but to compare "apples to apples", the ratio of shear to tensile strength for 7075 seems to be 65%).

To address complex load paths, CFRP must could be constructed with complex fiber orientation, for maximum strength and minimum weight. This would require individual strands to be oriented in the unique desired directions. A more practical, and less expensive, alternative, is to use CFRP with the familiar 0 degree/90 degree weave orientation. To maintain full strength in either direction (0 and 90 degrees), twice the material is required (the intended 0-degree plies, PLUS plies oriented at 90 degrees). And to handle loads at 45 degrees (as shear strength is weak), plies in both directions must be stronger (read: more- by a factor of the square root 2 = 1.41, or 41%, if my trigonometry is correct). So, potentially, to make a CFRP structure as "isotropic" as aluminum, would require about 2(for 90 degree loads) x 1.41(for 45 degree loads), or about 3 (2.82), times as much material as a "simple" anisotropic structure, and the marvel of a CFRP structure weighing 1/6 that of aluminum now becomes about half (0.47) as heavy. Still an impressive weight savings! And most assuredly, the design engineers will strive to minimize such wasteful excess.

(The ply orientation issue could have other solutions; one might be using 0-60-120 or 0-45-90 degree plies, rather than thicker 0-90 degree plies. The various solutions would result in slightly varying weights, and strengths in off-primary axis directions. It seems I've seen broken composites, and the jagged edge seemed to have fibers pointed in multiple directions- not sure if that is a result of the damage, or the inherent weave pattern of the composite fabric fibers).

Our visual observations of the 787 wing flex, does seem to substantiate this ball-park estimation, of roughly twice the wing flex of an aluminum wing. Regarding CFRP manufacturing and design allowances to handle the anisotropic limitations, Dow Chemical has an interesting article, which states "The key drivers for using CFRP are light weight (50 percent lighter than steel and 30 percent lighter than aluminum)", which would seem to indicate a lot of material is going into making composites act more isotropic (plus, probably some conservative design practices with the still relatively new technology). The Dow claim of only a 30% weight savings over aluminum (rather than our 50%-ish number above) seems to be proven in aviation- there seems to be no real-world weight savings of composite airplane versus aluminum, so far anyway. Perhaps CFRP manufacturing and design allowances to handle the anisotropic limitations (??)

Besides weight, excess material imposes, ah, excess cost. Boy, I thought the mechanical properties of CFRP was hard to find- the cost was even more proprietary and elusive. (Please see accompanying post at the top of this thread).

#2) "Does stiffness matter? Uh, well, er, "it ought to". But for now, let's say no"

One of the advantages of a "flexy" wing is absorbing gust loads and provides a better ride, and improved fatigue life for the rest of the airplane. (With a flexible wing, the overall upward velocity is not changed in response to a sustained updraft, but the rate of upward velocity change is slightly more gradual (and prolonged), so the vertical acceleration is smoother, and forces -and stresses- are lower). This allows components to be made less robust, and lighter.

On the other hand, one of the more alarming presumptions regarding the appearance of unusual/"excess" flex in a wing, regards susceptibility to flutter. And this might be where composites/CFRP shine. The X-29 forward-swept wing program was feasible because of the torsional stiffness of a composite wing. It would seem the 787 is benefiting from this as well, not that it is vulnerable to the inherent wingtip divergence the X-29 had, but still, that flexy wing needs to be resistant to torsion/bending coupling.

Jon's FlightBlogger website has the scoop on this too; some early 777 testbed work for variable camber effects, and his Better Know a Dreamliner - Part Two - ZA002 post yesterday, ("Airplane Two will have the second most hours of the six flight test aircraft and will first participate in the initial airworthiness and flutter clearance, as well as stability and control testing...High speed air testing is also expected to be a significant part of ZA002's aerodynamic check-out along with wing twist that will be measured"). With fly-by-wire controls, and tailored twist characteristics from CFRP construction, this should go smoothly. (Then again, how often have we heard "it's only software" :)

One last (thank goodness!) item to consider is fatigue life. This turned out to be disappointingly proprietary or buried exclusively in subscription trade magazines. The best I could find was Wikipedia CFRP, "Carbon fiber-reinforced polymers (CFRPs) have an almost infinite service lifetime when protected from the sun, but, unlike steel alloys, have no endurance limit when exposed to cyclic loading". So the "flexy" composite wings should not fatigue as aluminum would exposed to such large deflections. (One wonders about fuel and hydraulic lines though, but I suppose these are of a relatively small diameter such that bending will result in a low stress and strain).


Phil Bell said...

Okay- maybe I'm a little s-l-o-o-o-w...But: I caught on- finally.

The wacky looking wing flex, from especially the "fly away" view from 0:53-1:03. had me greatly perplexed. Surely the wing is not THAT flexible, right? What do you think:
Reuters video of ZA001 First Flight Takeoff

It finally dawned on me, after watching this probably 10 times, that it isn't a camera angle phenomenon, nor a weird lens problem*, but rather an "aspect ratio" problem. The video was probably shot for playback on HDTV wide-screen televisions (16:9 format)- we're watching the replays on computer monitors (which, I do have a 16:10 ratio LCD, but apparently the video encoding doesn't translate, so we're seeing it in 4:3 ratio, and the deflections are exaggerated by (16/9) / (4/3) = 1.33, or 33 percent. Re-watching the video, there are a couple of giveaways- the the vertical "oval" appearance of the engine inlets (0:22-0:28 and 0:33-0:38), and the ratio of the vertical tail height to the horizontal tail span (same 0:53-1:03).

This also probably explains the amazing "morphing" from a diamond-like planform (ala F-22), to a conventional swept wing silhouette during the flyover (0:42-0:46). Initially I had thought Boeing was trying to resorting to some aerodynamic tricks from the WW2 era Spitfire (the "Elliptical Wing Design" section is very informative reading- to a non-aerodynamicist, a diamond planform like the F-22 seems to roughly approximates an elliptical wing, and makes me wonder if that is where some of the apparent F-22 aerodynamic superiority to an F-15 comes in. I also suspect the F-22 engines are publicly underrated, but that's another story).

There's one more giveaway in the computer video- the black "shadow boxes" on the left and right edge- I measured them, and yup, including them yields 16:9 aspect ratio, excluding them results in a 4:3 ratio.

(Spitfire, Schmitfire- I get the "duh" award :! Still, earlier detection of this characteristic might have eliminated this lengthy investigation into composites, so, er, I guess there were some advantages to being slow in discovering the video aberration).

Phil Bell said...

This is an appeal for information from all our manufacturing engineering friends out there!!

Here's a 2004 Oak Ridge National Laboratory National Transportation Research Center paper, Automotive Lightweight Materials, which analyzes competing manufacturing methods for CFRP, with product costs of $30-130 per pound of finished product (Dodge Viper parts, rather than gold plated hammers, or some such. No "hammering" me over that reference- it's meant to imply the government did a good job of analyzing real-world reasonable volume production methods). Given that study is almost six years old now, let's say with more market penetration, the cost is down by half or so, to $15-65 per pound, for the various methods. (Then again, maybe not: "The market price of CFRP saw a 150% increase during 2005", per this article). However, this article puts the raw material at $10 per pound. Which illustrates the difficulty in figuring out prices (and material properties)- is that just the fiber, or the woven fabric, or the "composite" with epoxy, or the finished product after autoclaving and wasted material and typical defect rates are accounted for. For argument's sake, lets say the pre-autoclaved product is $15 per pound.

How about the cost of aluminum? Seems it has varied from $1.5 per pound during the pre 9-11 boom, to about half that now. But maybe the CFRP threat has got Alcoa concerned, Alcoa forms partnership to become low cost aluminum leader. Similar difficulties arise in correlating raw material to finished product costs- how much is milled away, what are typical reject rates, what does it cost to drill a hole, etc. Let's say the raw sheet that comes in is $1.50 per pound, or about 10% that of CFRP. Sounds good- but consider some complex parts have 90% of the material milled and machined away I'm told, and aluminum comes in at about the same price as composites- perhaps that helping with it's market penetration. (Then again, all those aluminum shavings can be recycled...). So for now, I'll settle for saying "CFRP costs more than aluminum", and "more CFRP costs more than less CFRP". Hopefully, a manufacturing engineer will post something to help us pin the $$ numbers down.

Phil Bell said...

It would seem Boeing has experimented with this long flexible wings before- the B-52 Load Alleviation and Mode Stabilization (LAMS) study from 1968. The testbed was flown above flutter speed, and used (analog!) fly-by-wire suppression of flutter modes, and demonstrated gust load alleviation, which was probably related to the change from high-altitude to low-level operations. (I had thought this to be a result of Vietnam era losses, but it appears the Gary Powers shootdown on May 1, 1960, was more a driver- talk about all dressed up and nowhere to go- the last B-52 rolled off the line on June 22, 1962). "Wingskins were changed in and the fuselage was strengthened in the 1970s".

Phil Bell said...

Well, as if we haven't had enough already (!?!), the pesky issue of those flexing hydraulic and fuel lines go my attention. Let's say the inner wing, between the fuselage and engines, is stiff enough to allow flex fittings to accommodate movement (the video would seem to indicate this). Let's look at the more flexible outer wing, and make a simple assumption- the entire wing bends up 90 degrees at the wingtip, and forms a circular arc (?are all arc's circular? I suppose they would be a spiral otherwise...) What is the radius of such an arc? Well, if the wingspan of the 787-8 is 197 feet, then one half the wing would be 98.5 feet long, which in hypothetical case, would represent a 90 degree arc length, which means the radius would be ? 2*pi*R = circumference for an entire circle (360 degree arc), so for 90 degrees, we would have (1/2)*pi*R = 98.5 feet, or R= 98.5 feet *2 / pi = 62.7 feet = 752.5 inches. A 1 inch o.d. tube, bent to this radius, would have what strain and stress? Let's consider a 1 inch long piece of tubing along that arc, which would have arcsine(1 inch/752.5 inch) = 0.001329 radians of curvature, and the length difference between the "neutral axis" (centerline) and inner surface would be 0.001329 radians * (diameter/2) = 0.000664 inches, so the strain would be = deflection/length = 0.000664/1.0 = 664 E-6. Young's modulus for a stainless steel line (Stainless Steel 304 Properties) is about 193 GPa, so the stress would be 664E-6 * 193E9 = 128 GPa. Since the yield strength is 290 GPa, our hypothetical 1-inch diameter line is fine.

This consideration was prompted by extreme wing bending, in the case of the 787, and extreme non-bending, in the case of the B-2 bomber, which was rumored to be dramatically stiff, due to it's thick wing. One consequence of such a stiff wing is purported to be a remarkable clean and leak-free airframe.

One other contributor to the relative lack of wing bending, for the B-2 versus 787, is wing loading:
The B-2 has a remarkably low aerodynamic wing load (376000 lbs MTOW, 5140 square feet of wing = 73.15 psf). The 787 is more typical (502500 lbs MTOW, 3501 square feet of wing = 143.53 psf, almost exactly double the B-2). Another helpful statistic is the somewhat shorter wingspan: 172 ft for the B-2, versus 197 ft for the 787.

Phil Bell said...

Hmmm, with a new appreciation of video display "aspect ratios", I re-examined the picture from the previous headline post, 787 First Flight, ZA001 takeoff.

Note that the engine inlets really are circular in this image, indicating proper aspect ratio representation- and yup, those wings really are curved upwards!

(The posted videos show that after landing and the speed bleed off, they really are straight, so there does seem to be a lot of real "flex" in them- and as others have pointed out, these were surely relatively light weights for first flight).

Phil Bell said...

As I noted to Jon, an interesting revealation about past claims regarding composites was brougt to mind by this exercise:

Over the past few years, I've had several bicycling friends tell me their composite-framed bikes "feel better" than their aluminum-framed bikes. I was always a little skeptical of that claim, thinking "composites are stronger, hence more rigid, hence rougher riding". But now I can see that the composite frames really could be flexier and more shock absorbent, while at the same time being lighter and stronger- I shoulda listened up! :)

fred said...

Excellent , phil ...

this could bring a whole new topic on its own :

"How to sort out what is new tech , most are not used to yet and Disruptive Garbage "

i am leaving for 2 days under a tent in the midst of desert ...
ideal to clean your body,soul and mind !

meanwhile , i wish to all readers a year 2010 full of love , success , health and only good things !

Gelukkige nuwe jaar (special Ken)
Gutes Neues Jahr(Special Julius)
Gelukkig nieuwjaar (Special Roel)
שנה טובה (whoever it concern)
Sona (Special Mr Shane)
كل عام وأنتم بخير (same as hebrew)
新年快樂 (missing chinese EA buyers)
あけましておめでとう (wing suppliers)
С новым годом (fantasy production plant)

Bonne Année et Bonne Santé a tous !

julius said...


just to make it easier:
You know those small boats (not those Gadfly is fond of), you can buy for checking ponds or rivers - one made of AL alloy and the other made of plastic. Which one are the lighter one? Naturally there are different types of "plastic"!

How does one find out the first signes of a delamination in a complex structure with even orthogonal layers?


P.S.: Are there any rules for the ultimate 150% tests (AL, plastic, including inside wires etc.)?
- 150%: everything ok
- 155%: first delamination (invisible)
- 160%....
or - 150,01%: total crash of the structure is allowed

gadfly said...


Actually, the "gadfly" has good memories of half of an aluminum drop tank in which my cousin and I paddled around the streams in Hansen Dam (a two-mile long flood control earth-filled dam, in northeast Los Angeles County near Roscoe and Sunland) back in the late 1940's. It was possibly from a wing tank on a P80 (since Lockheed is close by).

With two of us, it had maybe an eight-inch draft in the shallow streams and lakes (five), and easy to maneuver. I'm guessing it weighed no more than forty pounds. For us kids, the price was right . . . "Free"!

We left it behind for others to enjoy (it wouldn't fit on our bicycles).


baron95 said...


Regarding your inquiry on the effect of wing flew on fuel and hydraulic lines.... You should note that the 787 hydraulic architecture uses smaller diameter lines, because they operate at higher pressure (5,000 PSI), and a few other hydraulic drive motor choices.

Re Fuel lines, that is not an issue. Wing fuel tanks feed engines mounted below the wing - i.e. it is a very short run line ;) Fuel transfer, from one thank to the other, simply requires lines running under the fuselage (which does not flex).

So relax - and enjoy the up and down motion ;)

baron95 said...

As for CFRP's main benefits, it is simply to easy the grip that Alcoa et al have on aerospace materials and light weight transportation structures in general.

The competition between Aluminum and it's derivatives (like AuLi), high-strength/high-carbon steels and CFRP, will result in lowered costs (all things equal) and faster innovation.

Making it much more likely that your next car will have more non-traditional materials, and your plane too.

All in all, Happy New Decade to all.


Soccer Dad said...

You need to be very careful here in trying to make comparisons on strength between carbon fiber and aluminum. This is a subject which does not lend itself to simple comparisons and one of the reasons that you do not find a lot of information for the generic term CFRP is that your terminology is much too generic. To accurately determine the properties of a CFRP structure, you would need to know the specific fiber, the fiber form, orientations of the laminate and an inordinate number of different variables. When I try to explain composites to people, I use the analogy that we use to design the structure around the material limitations, but with composites, we can design the materials to achieve the structures that we want. Composites are incredibly complex, and comparisons to conventional construction are not easy to draw and that is where we most engineers get themselves in trouble with composites - if you design a composite structure like you design an aluminum structure, then you are destined to fail.

baron95 said...

Soccer Dad - I'm glad someone made that point.

BUT, lets not forget that Airbus has decided to go with a composite panel approach for the A350.

In a sense, they are using CFRP as black aluminum so to speak. (and I mean that just for the fuselage. Airbus has been using composites for much more complex structures (wing boxes, rear bulkhead, etc).

Phil Bell said...

Happy New Year !!

Happy New Decade, too !!

Whew- I am modestly confident the 2-1x's will be a big improvement over the 2-0x's!!

Best wishes to all, for a happy and healthy year ahead.

Black Tulip said...


Many thanks for this interesting piece on 'composites' versus aluminum. Us engineers love this stuff. Happy New Year.

Phil Bell said...

Hi BT,
I confess to giving it a layman's treatment, and as SoccerDad cautions, it is apparently a pretty complicated technology.

One correction to my opening post on this thread- after considering the effect of image "compression" in the horizontal axis, "deflections are exaggerated by (16/9) / (4/3) = 1.33, or 33 percent".

The overall vertical deflections are correct, regardless of horizontal image compression, and to be more precise: It's the radius of curvature that is exaggerated.

(Whew- even LOOKING at composites is a complicated affair !! :)

Phil Bell said...

Hi Baron,

Thanks for mentioning "black aluminum", I had come accross that term for the first time yesterday:

Getting To Know "Black Aluminum"
("CFRP—it’s been called black aluminum, and its design and fabrication has been described as a black art...")

Breakthroughs in Aerospace Composites Manufacturing

Although this article is a few years dated- references to the "7E7", perhaps the author has captured the connotation best:
"Up to now the use of composites has been mostly "black aluminum," that is, using
composites as a replacement for parts that were originally designed as aluminum parts."

Phil Bell said...

Hi SoccerDad,
Thanks for the cautionary note.

It would seem using composites as a direct "substitute' for aluminum, negates some of the potential weight savings (not really a "penalty", as the result is still -a little- lighter than aluminum, but still significantly heavier than a product that could be (at some greater expense in both engineering an manufacturing) built by "capitalizing" (lots of capital, I suspect! :) on the anisotropic (unidirectional) strengths of composite, and not bringing along the extra weight to make the composite assembly have aluminum's isotropic (non-directional) strength properties.

But in designing, and manufacturing, with anisotropic materials (uncompensted with alternate direction plies) apparently imposes a substantial cost penalty (although not a performance penalty), at least that's my explaination for why we don't see lighter GA airplanes...

The (much!) older, aluminum Cessna 182 (with an IO-540), has an empty weight of 1970 lbs

The composite Cirrus SR20 (with the lighter IO-360), has an empty weight of 2080 lbs.

(The newer, slicker Cirrus is 10 knots faster though).

Ideas/Opinions ??

Soccer Dad said...

I wouldn't classify Airbus's approach on the the 350 as black aluminum, my understanding of their approach had more to do with maintenance issue and in that respect, I would agree with the panel approach - certainly not the most efficient approach, but in the long run may prove correct - also, the panel approach eases some manufacturing issues as well - much easier to scrap a panel due to a blown bag during cure than a whole barrel.

As far as the weight efficiencies is to remember that all things do not scale linearly. Just because we make something from lighter materials does not necessarily mean that the overall structure will be lighter, we have to keep in mind that the materials perform differently and quite often, the use of a lighter, stronger material does not always equate to a lighter structure, especially when you consider that the various safety factors used in the design and analysis of composite structures are significantly higher than what is used for metallic structures - not necessarily in the basic safety factor, i.e. 1.5 for ultimate load cases, but in the generation of fundamental material allowables.

As I said earlier, it is extremely difficult to draw direct comparisons between an aluminum structure and a composite structure - composite materials can truly be tailored to the application.

I remember a old story about Burt Rutan, it was said that he never hired anyone with composite material experience from the big guys because all they knew how do to do was make black aluminum - not sure how accurate the story is, but black aluminum is a real problem in the industry.

Happy New Year Everyone.

baron95 said...

Hi Soccer Dad....

There were a number of factors on the A350 architecture selection. But in the end, remember that the current A350 is A350 version 4.0 or 5.0 depending on how you count.

Airbus tossed at Airlines a number of incremental approaches. From simply new engines on the A330 (A350 V1.0) to new engines plus reprofiled wings (v.2.0) to the current design that stuck. They made it wider.

Likely, there was not time for Airbus to establish a "barrel" supply chain in the time frame needed to counter the 787. Boeing had simply locked up all the large autoclaves, suppliers, CFRP layers, etc with exclusive contracts, not to mention a ton of controlling patents.

So Airbus got left with the choice of panels.

In my mind, the beauty of barrels, lies in them arriving at Everett fully stuffed with systems, electrics, etc for assembly. (Note, this is a personal opinion, I haven't read any analysis on this or even heard it mentioned as a key advantage).

This is simply impossible to do with panels. I think the final assembly labor costs for the A350 will be much higher than the 787.

Remember, Boeing is selling the 788 (the equivalent of a 77E) for the price of a 763ER.

(P.S. True Boeing has yet to receive the fuselage sections fully stuffed, but when that happens the savings will be there).

baron95 said...

And in more mundane piston news, several models of Rotax 912 piston engines now have TBOs of 2,000 hours, making them more competitive with the equivalent Lyc/Contis.

Soccer Dad said...

Your absolutely correct about the barrels and I do find it interesting in how and whether the supply chain will be able to integrate these systems consistently. Eventually this should be a very efficient supply chain and Airbus will be paying several penalties by taking the panel approach - one of the most difficult structures to make consistently out of any material, but particularly composites are panels with large radii - it is very difficult to make these consistently because there is more to keeping the geometry consistent because of the through the thickness effects on the geometry. Manufacturing and material consistency become increasingly important with this type of panel - especially if you are trying to integrate other pre-cured components into this assembly - in fact Boeing has had some issues in this regard on 787.

I wish Boeing and Airbus lots of luck with the programs, it's inspiring for those of us who have been in the rags and glue industry to finally see more mainstream support of composites. For too many years, the materials have been limited to the military and space industries and this has stunted the growth and knowledge of these materials. The proper use of composites is both incredible complex and simple at the same time - unfortunately to properly use composites, you have to forget everything you knew about isotropic materials. The other thing that one has to realize is, just like everything else, composites are not a magic bullet which can be used everywhere - there are poor applications for composites.

This should be an interesting few years to watch the A350 and the 787 programs mature and reach production - I'm sure we are going to learn a lot.

Soccer Dad said...

If you want an interesting look at the application of composites take a look at the Bell/Boeing V-22. That aircraft uses composites more extensively than anything I've ever run across - The boys at Bell truly understand composites - some of the things they are pulling off, I really don't think could be manufactured any other way.

Beedriver said...

Much of the use of composites has to do with the cost vs performance requirements of the project. My daughter with five years designing composites parts and now getting a masters says that composite parts that are truly very high performance and that are specifically designed for the application, However they typically carry a high cost with them. It is like using Titanium which is a great material but very expensive to work with.

therefore in an airplane like the 787 which will be heavily used, the weight savings and performance gains can be paid for by savings in the operation. thus it is very worth it to spend the extra money.
However in something like a light sport, the original purchase price dominates in the cost of ownership thus very expensive construction techniques are impossible to use. as a result you will see very little carbon and not very much composites used in the construction of many light sport airplanes. In most aircraft designs, you will see the right material used where it is most cost effective. aluminum is very good for structures like wings, as they are basically tapered regular section and flat sheets work well there. for cowlings composites possibly are best. you might find for the basic load carrying structure, for the fuselage for instance, you might use welded tubing.

This is true for all product design tasks . to do design well the cost benefit must be considered for any material that is used. No material is a cure all. for the 787 composites will allow a much lighter more efficient airplane and thus over the useful lifetime the use of carbon composite with more than pay for the higher initial cost.

In light sport aircraft or airplanes even like the cirrus very little carbon will be used as the extra cost cannot be justified for the weight savings.

In general The biggest problem with most product designers is that most engineers know very little about materials and where they should be used. For instance, the average engineer graduating from college has no idea what the difference is between acrylic and polycarbonate and where to use or not use each one. to be a great designer the engineer must intimately understand the properties of the materials they design with.

baron95 said...

A few items to keep in mind:

1 - Even the 787 and A350 are (will be) only 50% CFRP (by weight). So other materials from steel to aluminum to titanium will be there in droves.

2 - There is no question that the automotive industry will move very fast to aluminum, CFRP, in order to meet performance, safety and fuel efficiency requirements (a few market driven, most government mandated).

What will happen to the price of Aluminum, when you go from 4,000 planes/year using the stuff to 4,000,000 cars using it?

I think the Aluminum processes and supply chain is mature, with little chance for improvements in production price to compensate for increased demand.

CFRP production and its application tools (autoclaves, tape layers, etc), are much less mature. So I think they will benefit rapidly from the higher volumes of automotive applications.

Hard to predict, but it seems like Aluminum will become more expensive relative to CFRP technology in the next decade.

Car to watch is the next gen F10 M5. BMW has, for the most part, shunned Aluminum and COmposites - they have chosen to keep weight and cost down by high-carbon steels and shot blasted steel, and other techniques.

The buzz is that the new M5 will have large amounts of composites (e.g. roof), and composite or aluminum hood, trunk, doors.

I'm looking forward to the been counter/engineer material selection for that car as a trend setter.

Beedriver said...

Manufacturing processes and design can make a dramatic effect on production times. my republic seabee is still used to demonstrate how easily and cheaply an airplane can be built. By redesigning the airplane to use a large number of stretch formed skins which are very light and strong (think Grumman canoe) and redesigning the structure to use rivet squeezers instead of driven rivets.

the wings for instance are made of skin segments that are riveted together and then slipped over the spars where the rivets could be reached from the back and using corrugations formed in the skins there are only three ribs and only the center one requires driven rivets. the rivet count on the airplane went from 9650 to 3400 and the man hours to assemble the airplane went from 12,000 hours to 400 hours. Stretch formed aluminum is very cheap in volume and very strong.

an excerpt from an article follows.

During the war, Spencer worked for Republic Aircraft which had begun to look for a commercial design for sport flying that could be a viable post war business venture for the company. Some of Spencer’s colleagues at Republic remembered his Air Car design and after some negotiations Spencer sold the Air Car rights to Republic in December 1943. He was assigned to help convert the airplane to all metal construction. Along the way it became a three-place airplane with a bigger engine, metal hull from nose to tail, a tapered smooth skin cantilever wing and single strut-mounted wing floats. The airplane was renamed the Republic RC-1 “Thunderbolt Amphibian”. It was a good performer but the construction was very labor intensive and the costs began to skyrocket. The original intent was to market the airplane for $3,500 but price estimates projected a $12,000 sales price. To be market competitive the airplane’s structure was completely redesigned for low cost production and the aircraft became a four-place airplane for better utility. The rear seat in the RC-1 had been restricted to one passenger because the partially retracted main wheels were located in wells that protruded into both sides of the rear seat area. To provide space to seat two back seat passengers comfortably, the wheel retraction wells were eliminated and the main landing gear rotated up parallel to the hull. The design change slightly increased aircraft drag but reduced manufacturing costs and provided a fourth seat. Five pre-production prototypes were designated as RC-2s and renamed “Seabee’ and were extensively tested and demonstrated. The re-designated RC-2 was now a model of simplistic lightweight construction in labor and tooling costs. The rivet count, for example, was reduced from 9,650 to 3,440. The fabrication and assembly time was reduced from 2,500 man-hours to a phenomenally low 400. The design change from a tapered to a straight wing with simple spars and end cap ribs covered with beaded structural skins was a major contributor to these savings. The hull was essentially a full monocoque bulkhead and skin construction, thus eliminating the usual multiplicity of stringers and intercostals. The wing tip floats were made in two hydro-pressed longitudinal halves that were machine-riveted along their exterior seams. Full production aircraft were redesignated as RC-3s and started coming off the Republic production lines in the spring of 1946.

Beedriver said...

Woops, I used the wrong numbers for the reduction in man hours to build the Seabee amphibian airplane. it went from 2500 hours to 400 hours to build the airplane. not 12,000 hours.

Barry said...

Cover story on Sport-Jet in ... PilotMag

bill e. goat said...

Happy New Year to all!

Hi Fred,

"I am leaving for 2 days under a tent in the midst of desert ...
ideal to clean your body,soul and mind !"

Best wishes for the desert retreat, and new year!

(I think many will also be taking about two days to "clear their minds" after an evening of revelry!)

And thanks for the multi-lingual good wishes!

I was curious (and amazed!):

Gelukkige nuwe jaar (special Ken): Afrikaans
Gutes Neues Jahr(Special Julius): German
Gelukkig nieuwjaar (Special Roel): Dutch
שנה טובה (whoever it concern): Hebrew
Sona (Special Mr Shane): Irish
كل عام وأنتم بخير (same as hebrew): Arabic
新年快樂 (missing chinese EA buyers): Chinese
あけましておめでとう (wing suppliers): Japanese
С новым годом (fantasy production plant): Russian

I am sadly, mono-lingual, and sadly, not so good with even that!
So, I resorted to Bablefish, the on-line translator program.

(Interestingly, a web search for BableFish shows Alta-Vista references- a web browser I was fond of, but have not seen for a couple of years. It turns out Yahoo purchased them- interesting history- AltaVista was created by the old Digital Equipment Compay (DEC)).

Some hunting and pecking (greatly aided by Fred's hints) found a suitable translation for most languages (although Afrikaans required some thinking after an exploration of the languages geographically adjacent to Germany struck out!)

And Bablefish yielded this rather peculiar translation for the Japanese version of Happy New Year:
"Opening, you question with the me"
(I would suspect in the later hours of yesterday evening, much of what I said made about as much sense... :)

So, I found Google has a nifty (and at least for Japanese) and effective translator.

(And, I belatedly found, the Google translator even has a really cool "detect which language" feature- neat!

For our good friend Shane, I was able to find individual words in Irish for
Happy -> Sona
New -> Nua
Year -> Bliain

Cautious of the order of assembling words, I was not sure "Sona Nua Bliain" would work.

(As it turns out- maybe not so well, so I'll settle for this):

Ath bhliain faoi mhaise duit
"A Prosperous New Year to You"
(In consideration of Shane's forthcoming book- even more good wishes! :)

As Fred put it, and I couldn't say this any better in any language:

"I wish to all readers a year 2010 full of love , success , health and only good things !"

Bonne Année et Bonne Santé a tous !
(French for):
Happy New Year and Good Health to all

p.p.s.- For Baron and our friends in Brazil, (in Portuguese- heck, I guess it's for our friends in Portugal too!):

Feliz Ano Novo !!

A wish for all:

"Go mbeire muid beo ar an am seo arís"
(Irish for, "May we all be alive at this time next year!" :)

Irish Greetings and Phrases

baron95 said...

Hey B.E.G. - Feliz Ano Novo para você também. ;)

Tomorrow I leave Rio on AA 950 back to snowy CT and a full load of backed up work.

Moral of the story is: If you spend too much time at the Beech, you have to pay the Piper ;)

baron95 said...

What a sad piece the story on SportJet on Pilot Mag. Even by the standards of that publication (are there any?), this is a ungrounded cheerleading piece.

The only thing that is different about SportJet compared to the other PersonalJet attempts, is that it has less funding, and less to show for it than all the others. A crashed prototype and another one (non-conforming) starting flight "testing" is *HARDLY* distinguishing.

Diamond has had three flying prototypes for years, has a steady revenue stream, has experience, is going for *simple* and is still not done.

I wish SportJet well - I really do - but there is a difference between being an enthusiast and being foolish if you start taking in articles like the one linked above.

Other than Aviation Consumer, are there no critical thinking in the GA press?

Beedriver said...

Is there any news on Epic out there? I keep looking and can't find any news on it.

bill e. goat said...

Hi Baron,

Safe journey back to the US, and best wishes for a great 2010.

(Perhaps one of these days, folks will be taking that trip will be on a 787!)

airtaximan said...

just fyi,

one of the largest fleets in aviation used composites which do not require high pressure and high heat. No Autoclaves.

Bombardier is using this process on the Lear 85.

Phil Bell said...

New post will be up on Tuesday morning (sorry for the delay).

Phil Bell said...

New Year's Resolutions, involving a bathroom scale, gave inspiration for the new headline post!

(If you have difficulty reading it, please drop me a line at aviationcritic@gmail.com, and I'll email you the excel spreadsheet).